30 Thread support library [thread]

30.4 Mutual exclusion [thread.mutex]

30.4.4 Locks [thread.lock]

30.4.4.4 Class template shared_­lock [thread.lock.shared]

namespace std {
  template<class Mutex>
  class shared_lock {
  public:
    using mutex_type = Mutex;

    // [thread.lock.shared.cons], construct/copy/destroy
    shared_lock() noexcept;
    explicit shared_lock(mutex_type& m);        // blocking
    shared_lock(mutex_type& m, defer_lock_t) noexcept;
    shared_lock(mutex_type& m, try_to_lock_t);
    shared_lock(mutex_type& m, adopt_lock_t);
    template<class Clock, class Duration>
      shared_lock(mutex_type& m, const chrono::time_point<Clock, Duration>& abs_time);
    template<class Rep, class Period>
      shared_lock(mutex_type& m, const chrono::duration<Rep, Period>& rel_time);
    ~shared_lock();

    shared_lock(const shared_lock&) = delete;
    shared_lock& operator=(const shared_lock&) = delete;

    shared_lock(shared_lock&& u) noexcept;
    shared_lock& operator=(shared_lock&& u) noexcept;

    // [thread.lock.shared.locking], locking
    void lock();                                // blocking
    bool try_lock();
    template<class Rep, class Period>
      bool try_lock_for(const chrono::duration<Rep, Period>& rel_time);
    template<class Clock, class Duration>
      bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time);
    void unlock();

    // [thread.lock.shared.mod], modifiers
    void swap(shared_lock& u) noexcept;
    mutex_type* release() noexcept;

    // [thread.lock.shared.obs], observers
    bool owns_lock() const noexcept;
    explicit operator bool () const noexcept;
    mutex_type* mutex() const noexcept;

  private:
    mutex_type* pm;                             // exposition only
    bool owns;                                  // exposition only
  };

  template<class Mutex>
    void swap(shared_lock<Mutex>& x, shared_lock<Mutex>& y) noexcept;
}
1
#
An object of type shared_­lock controls the shared ownership of a lockable object within a scope.
Shared ownership of the lockable object may be acquired at construction or after construction, and may be transferred, after acquisition, to another shared_­lock object.
Objects of type shared_­lock are not copyable but are movable.
The behavior of a program is undefined if the contained pointer pm is not null and the lockable object pointed to by pm does not exist for the entire remaining lifetime ([basic.life]) of the shared_­lock object.
The supplied Mutex type shall satisfy the shared mutex requirements ([thread.sharedtimedmutex.requirements]).
2
#
[ Note
:
shared_­lock<Mutex> meets the Cpp17TimedLockable requirements ([thread.req.lockable.timed]).
— end note
 ]

30.4.4.4.1 shared_­lock constructors, destructor, and assignment [thread.lock.shared.cons]

🔗
shared_lock() noexcept;
1
#
Effects: Constructs an object of type shared_­lock.
2
#
Ensures: pm == nullptr and owns == false.
🔗
explicit shared_lock(mutex_type& m);
3
#
Requires: The calling thread does not own the mutex for any ownership mode.
4
#
Effects: Constructs an object of type shared_­lock and calls m.lock_­shared().
5
#
Ensures: pm == addressof(m) and owns == true.
🔗
shared_lock(mutex_type& m, defer_lock_t) noexcept;
6
#
Effects: Constructs an object of type shared_­lock.
7
#
Ensures: pm == addressof(m) and owns == false.
🔗
shared_lock(mutex_type& m, try_to_lock_t);
8
#
Requires: The calling thread does not own the mutex for any ownership mode.
9
#
Effects: Constructs an object of type shared_­lock and calls m.try_­lock_­shared().
10
#
Ensures: pm == addressof(m) and owns == res where res is the value returned by the call to m.try_­lock_­shared().
🔗
shared_lock(mutex_type& m, adopt_lock_t);
11
#
Requires: The calling thread has shared ownership of the mutex.
12
#
Effects: Constructs an object of type shared_­lock.
13
#
Ensures: pm == addressof(m) and owns == true.
🔗
template<class Clock, class Duration> shared_lock(mutex_type& m, const chrono::time_point<Clock, Duration>& abs_time);
14
#
Requires: The calling thread does not own the mutex for any ownership mode.
15
#
Effects: Constructs an object of type shared_­lock and calls m.try_­lock_­shared_­until(abs_­time).
16
#
Ensures: pm == addressof(m) and owns == res where res is the value returned by the call to m.try_­lock_­shared_­until(abs_­time).
🔗
template<class Rep, class Period> shared_lock(mutex_type& m, const chrono::duration<Rep, Period>& rel_time);
17
#
Requires: The calling thread does not own the mutex for any ownership mode.
18
#
Effects: Constructs an object of type shared_­lock and calls m.try_­lock_­shared_­for(rel_­time).
19
#
Ensures: pm == addressof(m) and owns == res where res is the value returned by the call to m.try_­lock_­shared_­for(rel_­time).
🔗
~shared_lock();
20
#
Effects: If owns calls pm->unlock_­shared().
🔗
shared_lock(shared_lock&& sl) noexcept;
21
#
Ensures: pm == sl_­p.pm and owns == sl_­p.owns (where sl_­p is the state of sl just prior to this construction), sl.pm == nullptr and sl.owns == false.
🔗
shared_lock& operator=(shared_lock&& sl) noexcept;
22
#
Effects: If owns calls pm->unlock_­shared().
23
#
Ensures: pm == sl_­p.pm and owns == sl_­p.owns (where sl_­p is the state of sl just prior to this assignment), sl.pm == nullptr and sl.owns == false.

30.4.4.4.2 shared_­lock locking [thread.lock.shared.locking]

🔗
void lock();
1
#
Effects: As if by pm->lock_­shared().
2
#
Ensures: owns == true.
3
#
Throws: Any exception thrown by pm->lock_­shared().
system_­error when an exception is required ([thread.req.exception]).
4
#
Error conditions:
  • (4.1)
    operation_­not_­permitted — if pm is nullptr.
  • (4.2)
    resource_­deadlock_­would_­occur — if on entry owns is true.
🔗
bool try_lock();
5
#
Effects: As if by pm->try_­lock_­shared().
6
#
Returns: The value returned by the call to pm->try_­lock_­shared().
7
#
Ensures: owns == res, where res is the value returned by the call to pm->try_­lock_­shared().
8
#
Throws: Any exception thrown by pm->try_­lock_­shared().
system_­error when an exception is required ([thread.req.exception]).
9
#
Error conditions:
  • (9.1)
    operation_­not_­permitted — if pm is nullptr.
  • (9.2)
    resource_­deadlock_­would_­occur — if on entry owns is true.
🔗
template<class Clock, class Duration> bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time);
10
#
Effects: As if by pm->try_­lock_­shared_­until(abs_­time).
11
#
Returns: The value returned by the call to pm->try_­lock_­shared_­until(abs_­time).
12
#
Ensures: owns == res, where res is the value returned by the call to pm->try_­lock_­shared_­until(abs_­time).
13
#
Throws: Any exception thrown by pm->try_­lock_­shared_­until(abs_­time).
system_­error when an exception is required ([thread.req.exception]).
14
#
Error conditions:
  • (14.1)
    operation_­not_­permitted — if pm is nullptr.
  • (14.2)
    resource_­deadlock_­would_­occur — if on entry owns is true.
🔗
template<class Rep, class Period> bool try_lock_for(const chrono::duration<Rep, Period>& rel_time);
15
#
Effects: As if by pm->try_­lock_­shared_­for(rel_­time).
16
#
Returns: The value returned by the call to pm->try_­lock_­shared_­for(rel_­time).
17
#
Ensures: owns == res, where res is the value returned by the call to pm->try_­lock_­shared_­for(rel_­time).
18
#
Throws: Any exception thrown by pm->try_­lock_­shared_­for(rel_­time).
system_­error when an exception is required ([thread.req.exception]).
19
#
Error conditions:
  • (19.1)
    operation_­not_­permitted — if pm is nullptr.
  • (19.2)
    resource_­deadlock_­would_­occur — if on entry owns is true.
🔗
void unlock();
20
#
Effects: As if by pm->unlock_­shared().
21
#
Ensures: owns == false.
22
#
Throws: system_­error when an exception is required ([thread.req.exception]).
23
#
Error conditions:
  • (23.1)
    operation_­not_­permitted — if on entry owns is false.

30.4.4.4.3 shared_­lock modifiers [thread.lock.shared.mod]

🔗
void swap(shared_lock& sl) noexcept;
1
#
Effects: Swaps the data members of *this and sl.
🔗
mutex_type* release() noexcept;
2
#
Returns: The previous value of pm.
3
#
Ensures: pm == nullptr and owns == false.
🔗
template<class Mutex> void swap(shared_lock<Mutex>& x, shared_lock<Mutex>& y) noexcept;
4
#
Effects: As if by x.swap(y).

30.4.4.4.4 shared_­lock observers [thread.lock.shared.obs]

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bool owns_lock() const noexcept;
1
#
Returns: owns.
🔗
explicit operator bool() const noexcept;
2
#
Returns: owns.
🔗
mutex_type* mutex() const noexcept;
3
#
Returns: pm.